That is definitely an American buttercream, a combination of butter, shortening and powdered sugar. I like seriouscakes recipe, www.seriouscakes.com. You don't need a crusting buttercream, but definitely an American buttercream. If it is going to be outdoors, you'd want at least 1/3 of your fat to be hi-ratio shortening, like Alpine or Sweetex.

You could do that with Swiss Meringue Buttercream, but if it gets hot it will not be very stable.

AIt doesn't need to be crusting. Those rosettes are a lot of icing and I think that would be too sweet. PM me and I will send you my recipe. It's pretty heat-friendly. You should definitely keep the cake cold for as long as you can before putting it under the tent.